A. Field of the Invention
This invention relates generally to the field of telecommunication and the processes by which voice and digital data are transmitted over telephone networks to and from a customer's premises. More particularly, the invention is directed a telephone line conditioner device that is installed in the network between the telephone company network and the lines leading to the customer premises. Basically, the telephone line conditioners described herein are devices to improve the transmission quality of the telephone lines. Line conditioners perform several functions, including amplifying and equalizing the transmission line between the network and customer premises. Because line conditioners are a type of signal repeater located at an intermediate point along a two-wire transmission line, they typically separate the full-duplex (i.e., simultaneous and bidirectional) two-wire transmission lines into two two-wire simplex (unidirectional) circuits. The devices thus contain interface circuitry, commonly known as hybrids, to convert the two-wire full-duplex circuit to two simplex circuits (or four-wire circuit), and to convert the four-wire circuit back to a two-wire circuit. Hybrids originally used transformers to connect the two-wire and four wire circuits. Electronic hybrids were also developed to split and recombine the circuits. The line conditioners amplify and equalize the signals in the simplex circuits between the hybrids.
In a typical POTS application, a line conditioner is located at a customer's residence. The device has two pairs of terminals: one pair is connected to the customer's equipment (the "customer side" or "east side"); the other pair is connected to the phone company's equipment (the "line side" or "west side"). Internally, as indicated above, the line conditioner device has circuitry for the two distinct signal paths: one for signals from the customer side to the line side (or an "east-west" circuit) and one for signals originating from the line side to the customer side (or a "west-east" circuit). Because each pair of terminals is used both for incoming and outgoing signals, one problem associated with line conditioning circuits is that an outgoing signal being directed towards the two-wire system is also simultaneously detected as an input and is retransmitted in the direction from which it originated, thereby causing an echo. Such echos are highly undesirable because they are distracting to the speaker, and can seriously degrade the performance of communications devices such as computer modems and facsimile machines. Effective line conditioning devices must therefore be capable of minimizing echos.
B. Description of the Prior Art
One prior art technique for preventing or suppressing echos is to allow communications in only one direction at a time. Communications formats of this type are commonly referred to as half-duplex. The conditioners inhibit or mute signals from one side when signal energy is detected from the other side. This has the effect of eliminating the possibility of signals returning to the source, but also prevents an individual from hearing speech, or any other sounds, originating from the other party as long as the individual is speaking. The technique also causes dropouts during the switching of the circuits which tends to disrupt conversations. Additionally, this echo suppression technique must be disabled when simultaneous bidirectional communications are required, as is the case with computer modems.
Perhaps the most widely used technique of echo cancellation utilizes a feedback tap from the hybrid interface circuitry to cancel the echo signal, as shown in FIG. 3. The hybrid is adjusted so that the feedback signal at node N.sub.0 closely matches the echo signal at node N.sub.1 so the feedback signal can be utilized to fully cancel the echo signal. This type of adjustment is commonly referred to as "balancing" the hybrid. The optimum echo cancellation is obtained when the ratio of R.sub.FB /Z.sub.var exactly matches the ratio of R.sub.OUT /Z.sub.line because in this "balanced" state, the voltage at the output node N.sub.1 is equal to the voltage at the feedback tap N.sub.0, thus facilitating echo signal cancellation without impeding the propagation of west-east signals received at the node N.sub.0.
The value of Z.sub.var which results in the best balance of the hybrid is of course equal to (or proportional to) Z.sub.line. However, because Z.sub.line is a complex parameter derived from transmission line characteristics involving reactive and resistive impedances distributed along the transmission line, it is usually modeled as a simpler circuit using lumped circuit elements. In this way, a circuit realization for Z.sub.var that corresponds generally to the estimated Z.sub.line is more easily implemented. Various transmission line models have been used within four-wire to two-wire hybrids to reduce echos. Prior art devices which use this type of echo cancellation balance the hybrid by adjusting variable resistors, capacitors and/or inductors in the presence of a tone, typically 2 kHz, which is applied to the hybrid to mimic a transmitted signal. The amount of signal cancellation is then measured using one or more voltage comparators, correlators, or other comparison circuits to achieve the best echo signal cancellation of which the device is capable.
The inventors named herein discovered that the use of a narrow-band signal to balance the hybrid does not result in adequate echo cancellation when the communications circuit is in operation because the impedance of the line varies significantly as a function of frequency. The use of a single tone to balance the hybrid only ensures that frequencies only in that part of the spectrum will be canceled effectively. Previously, this single-tone technique has resulted in acceptable echo cancellation performance for voice communications. Recently, with the wide use of data communications devices, a need for a more accurate balancing has arisen.
Additional prior art line testing devices (known as a "test set", typically used by the phone company technician) use white noise for the calibration signal, but this is also not a good method, as it requires an oscillator or noise generator to make the signal, thus requiring additional components. Additionally, the noise power spectrum has a flat amplitude, and is not weighted to reflect the transmissibility of the voice band.
The present invention uses multitone broad-band measuring of the line impedance properties to provide for a more complete and comprehensive echo reduction over the entire voice band spectrum. Furthermore, the present device accomplishes this task without introducing additional components necessary just for the line balancing. Further, the amplitude of the signals is weighted to reflect the full voice band.
Two prior art patents that disclose line conditioner devices include U.S. Pat. No. 4,961,218 and 5,504,811.